1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. The widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines are increasing the need for gasoline "octane", or knock resistance of the gasoline component. The catalytic reforming unit must operate at higher severities in order to meet these increased octane needs. This trend creates a need for more effective reforming catalysts and catalyst combinations.
The multi-functional catalyst composite employed in catalytic reforming contains a metallic hydrogenation-dehydrogenation component on a porous, inorganic oxide support which provides acid sites for cracking and isomerization. Catalyst composites comprising platinum on highly purified alumina are particularly well known in the art. Those of ordinary skill in the art are also aware of metallic modifiers, such as rhenium, iriduim, tin, and germanium which improve product yields or catalyst life in platinum-catalyst reforming operations.
The composition of the catalyst, feedstock properties, and selected operating conditions affect the relative importance and sequence of the principal reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Naphthene dehydrogenation takes place principally in the first catalyst zones, while hydrocracking is largely accomplished in later catalyst zones. High yields of desired gasoline-range products are favored by the dehydrogenation, dehydrocyclization, and isomerization reactions.
The performance of catalysts employed in the catalytic reforming of naphtha range hydrocarbons is measured principally by three parameters:
(1) Activity is a measure of the ability of the catalyst to convert hydrocarbon reactants to products at a designated severity level, with severity level representing a combination of reaction conditions: temperature, pressure, contact time, and hydrogen partial pressure. Activity typically is designated as the octane number of the pentanes and heavier ("C.sub.5 .sup.+ ") product stream from a given feedstock at a given severity level, or conversely as the temperature required to achieve a given octane number.
(2) Selectivity refers to the yield of petrochemical aromatics or C.sub.5.sup.+ product from a given feedstock at a particular activity level.
(3) Stability refers to the rate of change of activity or selectivity per unit of time or of feedstock processed. Activity stability generally is measured as the rate of change of operating temperature per unit of time or of feedstock to achieve a given C.sub.5.sup.+ product octane, with a lower rate of temperature change corresponding to better activity stability, since catalytic reforming units typically operate at relatively constant product octane. Selectivity stability is measured as the rate of decrease of C.sub.5.sup.+ product or aromatics yield per unit of time or of feedstock.
Higher catalyst activity is required to meet the need for high octane gasoline components at reasonable operating conditions, and improved catalyst selectivity becomes more important as higher operating severities reduce the yield of desired product.
Higher operating severities also accelerate the deactivation of the catalyst. The principal cause of deactivation of a dual-function catalyst in a catalytic reforming operation is the aforementioned formation of coke on the surface of the catalyst. Alternative approaches to reactivation of the catalyst are well known to those skilled in the art. Regeneration of the catalyst may be carried out during a periodic shutdown of the unit, i.e., a "semiregenerative" operation, or by isolation and regeneration of individual reactors, i.e., a "swingreactor" system. In a "continuous" operation, catalyst is withdrawn by means of a slowly moving bed, regenerated, reactivated, and returned to the reactors. The "hybrid" system is a combination of regeneration techniques, in which a reactor associated with continuous catalyst regeneration is added to an existing fixed-bed system. The reactants may contact the catalyst in individual reactors in either upflow, downflow, or radial flow fashion, with the radial flow mode being preferred.
The problem facing workers in this area of the art, therefore, is to develop catalyst systems with improved activity, selectivity, and stability for a variety of feedstocks, product requirements, and reactor systems. This problem has become more challenging due to the aforementioned increase in required catalytic reforming severity. Multi-catalyst-zone systems, in which different catalyst composites are employed in the sequential zones of the reactor system, are of increasing interest as a solution to the problem. The activity, selectivity, and stability characteristics of individual catalyst composites are complementary to the specific reactions occurring in the different zones of the multi-zone system.